1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a laser irradiation method.
2. Description of the Related Art
In recent years, extensive research has been conducted on laser crystallization methods used to crystallize a semiconductor film (for example, an amorphous semiconductor film) formed over a glass substrate through irradiation of the semiconductor film with laser light.
Crystallization of a semiconductor film is performed in order to increase carrier mobility through crystallization of the semiconductor film. The crystallized semiconductor film is used, for example, in a thin film transistor (hereinafter described as a TFT). When a semiconductor film formed over a glass substrate has been crystallized, an active matrix display device (for example, a liquid crystal display device or an organic EL display device) can be manufactured through formation of a TFT for use in a pixel and TFT for use in a driver circuit, using the semiconductor film.
Methods for crystallizing a semiconductor film, other than the laser crystallization method, include a thermal annealing method which uses an annealing furnace and a rapid thermal annealing method (RTA method). However, these methods need treatment at a high temperature greater than or equal to 600° C. Because of this, use of a quartz substrate that can withstand treatment at high temperature is necessary and causes manufacturing costs to increase. In comparison with these methods, since heat can be absorbed only by a semiconductor film in the laser crystallization method, the semiconductor film can be crystallized without increasing the temperature of the substrate very much. Therefore, a material with low heat resistance, such as glass or plastic, can be used for the substrate. Accordingly, an inexpensive glass substrate that can be easily processed with a large area can be used, and the production efficiency of the active matrix display device increases considerably.
Conventionally, a method using an excimer laser which is a pulsed laser has been used as the laser crystallization method. Since a wavelength of an excimer laser belongs to an ultraviolet region, the laser can be efficiently absorbed by silicon and heat can be selectively applied to silicon. When an excimer laser is used, for example, laser light with a rectangular shape (for example, a rectangular shape with an area of 10 mm×30 mm) emitted from a laser oscillator is processed by an optical system into laser light with a linear cross section (for example, a linear cross section with an area of several hundreds of micrometers×300 mm). Then, a semiconductor film is irradiated with the linearly processed laser light while the laser light is moved relative to the semiconductor film, whereby the whole semiconductor film is crystallized sequentially. With the direction, in which the laser light is moved, being perpendicular relative to the laser light, crystallization efficiency increases.
In comparison, in recent years, a technology for manufacturing a semiconductor film including a region with crystals of much larger grain size (also referred to as a large grain crystal region) than crystals of a semiconductor film crystallized by an excimer laser has been developed, in which the semiconductor film is irradiated with a continuous-wave (CW) laser or a pulsed laser with a repetition rate of 10 MHz or more to be processed into linear laser light, while the laser light is moved relatively to the semiconductor film. When this large grain crystal region is used as a channel region of a TFT in manufacturing the TFT, energy barriers against carriers (electrons or holes) decrease because fewer grain boundaries exist in the direction of the channel. As a result, the manufacture of a TFT that has a mobility of several hundreds of cm2/Vs becomes possible. (For example, see Patent Document 1: Japanese Published Patent Application No. 2005-191546).